1. Field of the Invention
The present invention related to optical elements and its producing method and in particular, related to optical elements having a microstructure with extraordinary high aspect ratio and preferable producing method of it.
2. Description of Related Art
In the recent years, in optical pick up device field developing rapidly, optical elements such as extra high accurate objective lenses are utilized. Molding the materials like plastic and glass into such optical elements by metal die, uniform products in its configuration can be made rapidly therefore, such metal molding are regarded to be suitable for mass production of such optical elements.
Also, as recent pick up devices, the devices capable of conducting recording and/or reproducing high density information on HD DVD (High Definition DVD) and BD (Blue-ray Disc) using beam from shorter wave semiconductor laser are being developed. To improve the aberration of optical system, optical elements with various ideas are provided. Though such optical pick up devices capable of conducting recording and/or reproducing of high density information, it is needed to maintain the ability of conducting recording and/or reproducing for existing CDs and DVDs that have been supplied in large quantity so far. In such optical pick up devices capable of conducting recording and/or reproducing compatibly, there is sometimes used a wave plate giving a phase difference to make optical systems common.
Japanese Published Utility Model Application Jitsukaisho 61-155801 (JP61-155801U), discloses that an ordinary wave plate is made by grinding expensive material such as crystal. Further, it discloses that a method using a grid is suggested in addition to the crystal grinding method, because high density surface relief grating shows the character of birefringence, thus it is known as a type of wave plate having so-called microstructure.
Structural birefringence means birefringence generated by the directional properties of microstructure. For instance, micro period structure (so-called line and space structure) in which flat plates having no birefringence with different refractive index are laid in parallel within a period smaller enough (<λ/2) than wavelength λ of light is known to generate birefringence (Refer to Principle of Optics, Max Born and Emil Wolf, PERGAMON PRESS LTD.).
Refractive index np of light whose direction of polarization is parallel to the groove and refractive index nv of light whose direction of polarization is perpendicular to the groove are as follows respectively:np=(tn12+(1−t)n22)1/2  (1)nv=1/(t/n12+(1−t)/n22)1/2  (2)n1 and n2 are a refractive index of substance (line) in which microstructure is molded and a refractive index of substance(space) respectively. Also t is duty rate of micro period structure and is:t=w1/(w1+w2)  (3)meanwhile, w1 and w2 are the width of substance(line) constructing micro period structure and the width of substance(space) filling the groove respectively. While the birefringence contained in crystal and calcite is indigenous to the substances and cannot be changed, the birefringence of micro period structure can be controlled easily by altering the material or the shape of it. Also, the phase difference (retard amount) Re between a light having direction of polarization parallel to the groove and a light having direction of polarization perpendicular to the groove will be:Re=(np−nv)d  (4)provided that the height (depth of the groove) of birefringence of micro period structure is d.
According to these formulas, the phase difference (retard amount) Re can be changed by making the duty rate t of the birefringence of micro period structure and the height of birefringence (the depth of groove) d variable.
Therefore, for instance, when a quarter-wave plate for 400 nm laser beam is needed to be formed as a optical element, using plastic material having an refractive index of around 1.5 at an ordinary temperature, if the widths of line and space are 100 nm and 90 nm respectively, the height of structure d has to be 1200 nm. In this case, since the aspect ratio will be around 12, such microstructure is difficult to form.
AS FIG. 1(a) and FIG. 1(b) show, “Aspect Ratio” is the value indicated by B/A provided that the width of convex or concave is represented by A and the depth or the height is represented by B in the microstructure. “Microscopic shape” means the form in which the value A is not more than 10 μm.
Contrarily, as the optical element disclosed in JP61-155801U, it is considered that it is possible to give a phase difference of ¼ by forming microstructures with half aspect ratio on each of the plane of incident side and the plane of emergency side so that a ray transmits these two microstructures. However JP61-155801U discloses that microstructures are made on the plane of incident side and the plane of emergency side by pressing single bulk material however, the specific method to make the optical elements is not clarified. In particular, it is considerably difficult to form microstructures in two directions on single bulk material.
The wave plate having microstructure mentioned above has line and space structures which line up within a period of not more than ½ wavelength of the light which is passing through the wave plate. Therefore, the problem is that, in ordinary injection molding, the materials are difficult to go into each steps of a microscopic shape made on the die, simply by injecting the melting plastic, thus the transferring of microscopic shape cannot be done accurately. If the microstructure cannot be formed as planned, because of poor transferring quality (due to dull material), the optical character will be deteriorated and the optical pick up device with such optical element may cause recording errors. Therefore, though various ideas had been tried, it is difficult to eliminate the dullness completely in the conventional method.
On the other hand, Unexamined Japanese Patent Application No. 2002-220241 (JP2002-220241A) discloses the method to form optical element having micro pattern on its surface by pressing glass material while it is softened by heat.
However, forming microstructure with the aspect ratio of around 0.2 wherein the width is approx. 10-50 MM and the height is approx. 20-10 MM, on the glass surface is the utmost the art described in JP2002-220241A can do. This is because the coefficient of elasticity of inorganic glass is as high as 70 GPA at ordinary temperature, and therefore the glass material doesn't go deep into of microstructure smoothly, even if the glass surface is pressed by heated die with extremely high pressure of 3000N, resulting in the utmost of forming microstructure with aspect ratio of 0.2. Therefore, the molded products having microstructure with aspect ratio of not less than 1 may exist as prototypes however, industrial products with uniformity in their shapes do not exist yet.
Meanwhile, JP61-155801U discloses an optical element having microstructures on its plane of incident side and plane of emergency side however, how the microstructure with high aspect ratio is formed is not clarified specifically and only description says that a stamper is used to press both surfaces of acryl board with heated die to form the microstructure.
The optical element described in JP61-155801U has a problem that is that foreign matters such as dust, water and oil can easily adhere to microstructure, because microstructures are formed on each of plane of incident and plane of emergency of optical element and in particular, liquid can easily go deep inside by a capillary phenomenon and is difficult to remove. Also another problem is that microstructure with high aspect ratio can easily be damaged by external force.